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Predictions from the Eta model in which analysis errors over the data sparse eastern Pacific Ocean induce forecast errors over the western United States in the shorter range (12-24 h) and over the eastern United States in the longer range (36-48 h) are discussed. The forecast errors that first appear in a trough (ridge) in middle and upper tropospheric westerlies over the west coast of the United States propagate eastward with the trough (ridge). The errors amplify when they reach a baroclinic zone and affect the development of predicted surface storms. A procedure that may help forecasters judge relative accuracy of two forecasts valid at the same time, one of the shorter range than the other, is also presented.

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